Section 5: Analyzing Solutions—Titrations

Figure 8-8. Titration

A buret (or burette) is used for titrations, a method to determine the amount of solute in a solution. This apparatus allows chemists to add a solution drop by drop until the titration reaches its end point.

Using a volumetric flask, it is straightforward to create a solution of a particular concentration. But if we want to find the concentration of an unknown solution, we have to analyze it. This could be to determine the concentration of a solution formed from an industrial reaction or it could be to analyze a sample of water from the environment for toxins. The preferred method for chemists who want to analyze a solution to determine its concentration is called a "titration." A titration is a form of volumetric analysis because the only thing that gets measured is the volume of the solution that is added during the titration experiment.

To perform a titration, we need to find a reaction that the solute (S) to be analyzed will undergo with another molecule (T), which we can add or titrate into the solution. These two chemicals should react together to make a product (P). In an ideal setup, we want to find a way to make sure we add exactly enough T to the solution to react with all of the S. In other words, a titration is just a controlled chemical reaction where there is no limiting reagent, because it should end exactly when both S and T are consumed by the reaction. The point when this reaction is complete is called the "end point" of the titration.

S + T → P

Solute + Titrant → Product

So, to do a titration, a solution of T is slowly added drop by drop into a solution of S using a buret (or burette), which marks the exact volume of the solution of T that is added. (Figure 8-8) Before starting the titration, a solution of T is prepared where the concentration is known exactly. So, once exactly enough of solution T has been added to react with solution S, the stoichiometry of the balanced chemical reaction can be used. Since the volume of solution T is known, as is its concentration, the number of moles of S that were in the solution that was being titrated is now known. However, the tricky part of a titration is being able to tell exactly when the reaction is complete.

A Practical Titration: Vitamin C in Grapefruit Juice

How much vitamin C is in grapefruit juice? We use a titration to figure this out. Here's how that works. In this case, the solution is the grapefruit juice, and the dissolved solute we are interested in is the vitamin C...

A Practical Titration: Vitamin C in Grapefruit Juice

How much vitamin C is in grapefruit juice? We use a titration to figure this out. Here's how that works. In this case, the solution is the grapefruit juice, and the dissolved solute we are interested in is the vitamin C.

To start, we need another molecule that we can add to the grapefruit juice that will react with the vitamin C. We'll use iodine (I2). When we add iodine to vitamin C, it produces iodide (I-) as shown in this equation:

To titrate the grapefruit juice, we add a known amount of iodine (I2) drop by drop until all the vitamin C is gone and the reaction ceases. Then, we figure out how much iodine we added to determine the amount of vitamin C. Simple, right? But how will we know when the reaction has stopped and we have reached the end point?

We need an indicator. The trick we need to know for this titration is that if there is even the smallest amount of excess iodine around, it can react with iodide to form triiodide (I3-). Triiodide turns dark purple when it is in the presence of starch. Starch is a complex sugar common in nature (potatoes, for example, are made almost entirely of starch). So, we add starch to our grapefruit juice. Now, if even the smallest fraction of a drop is added past the end of the reaction, the color purple will appear in our sample. Ideally, it would be the faintest color purple possible that the eye can see. If it were dark purple, it might mean that we added excess iodide, which would ruin the data from the experiment.

Indicators

For the titration described above to work, we must have some way of determining exactly when the reaction has stopped occurring, since molecules are too small to monitor with the human eye. Therefore, it would be perfect if there were a visual change when the exact amount of solution T has been added to completely react with solution S. If either S or T is a colored compound, we can keep adding the solution of T drop by drop until the color disappears; unfortunately, most of the chemicals used in titrations are colorless. Therefore, a common way to determine the point at which the chemical reaction is complete is to use a substance called an "indicator." An indicator is a molecule that changes color to indicate when the reaction is over. When we see the color change, we know that exactly enough of solution T has been added to react with solution S.

So long as the indicator changes color when the reaction is over, it can be useful. There are many things to consider when picking an indicator. We will further address indicators in Unit 10.

Glossary

End point

The point in a titration where the reaction ceases to occur. It indicates that all of the reactant in the flask is used up.

Indicator

A substance that undergoes a change, usually in color, that indicates the endpoint of a titration.

Titration

A technique for measuring the concentration of a solute in a solution by carefully measuring how much of a second molecule, called the "titrant," reacts with it. As titrant is added to the solution, the reaction takes place until the solute is used up; the amount of titrant required to reach this point (see End Point) can be used to deduce how much solute was in the solution.

End point

The point in a titration where the reaction ceases to occur. It indicates that all of the reactant in the flask is used up.

Indicator

A substance that undergoes a change, usually in color, that indicates the endpoint of a titration.

Titration

A technique for measuring the concentration of a solute in a solution by carefully measuring how much of a second molecule, called the "titrant," reacts with it. As titrant is added to the solution, the reaction takes place until the solute is used up; the amount of titrant required to reach this point (see End Point) can be used to deduce how much solute was in the solution.